@Bonnie-Tuomala I am writing this from my mobile and out of my mind, so I did not test it.
Open replace dialog, mark regular expression, which is in the left corner, and put the following
into the find what

(.*{200})(\R)

And in replace with

\1__@__\2

@ needs to be replaced with 410 spaces.
Easiest way would be doing it for one line manully and copy the spaces between \1 and \2